This invention relates in general to passive magnetic bearings and in particular to superconducting bearings.
Passive magnetic bearings offer significant advantages over active magnetic bearings and conventional mechanical bearings, especially for applications requiring moderate bearing stiffness and load capacity. Passive magnetic bearings utilize inherent magnetic forces to suspend their rotors instead of contact forces or electrically-supplied forces. Operating with almost no friction, passive magnetic bearings have lower drag losses than do active magnetic and conventional mechanical bearings. Further, passive magnetic bearings do not require lubrication or bulky, complex electronics.
A conventional passive magnetic bearing is disclosed in U.S. Pat. No. 3,614,181 issued to Meeks on Oct. 19, 1972. Secured to a shaft is a first plurality of radially-polarized magnets, which are arranged in alternating polarity. Surrounding the first plurality of magnets is a second plurality of radially polarized ring magnets also arranged in alternating polarity. Uniform radial repulsive forces between the two pluralities of magnets cause the shaft to be suspended. Still, the shaft can be rotated by a minimal amount of force. However, according to Earnshaws Theorem, total permanent magnet levitation is inherently unstable and, hence, not practical for use in bearing systems.
Passive magnetic bearings can also be made of superconducting material. A thrust bearing is formed by placing a magnet above a disk made of a superconducting material cooled below its critical temperature T.sub.C, and a journal bearing is formed by placing a cylindrical magnet within a hollow cylinder made of superconducting material cooled below its critical temperature T.sub.C. For an example of a superconducting journal bearing, see Gyorgy et al. U.S. Pat. No. 4,797,386.
Passive magnetic bearings made of Type I superconducting materials are thought to experience rotor stability problems. Type I superconductors have the ability to exclude all or some of the magnetic flux from applied magnetic fields less than some critical field H.sub.C. Known as the "Meissner Effect", this exclusion of flux is believed to be caused by persistent currents that flow at the surface of the Type I superconductor. When excluded, the flux flows around the superconductor, providing a lifting force. This lifting force causes the magnet to be levitated above the Type I superconducting disk or within the Type I superconducting cylinder. To stabilize the rotor of this Type I superconducting bearing, the bearing can generally employ either a mechanical rotor support (see, e.g., Buchold U.S. Pat. No. 3,026,151) or a dished structure that provides a gravitational minimum (see, e.g., Emaile et al. U.S. Pat. No. 3,493,274).
Magnetic bearings made of Type II superconducting materials exhibit the rotor stability lacking in the conventional magnetic and Type I superconducting bearings Type II superconductors exclude flux from applied magnetic fields less than a first critical field H.sub.C1 and conduct flux for applied magnetic fields in excess of a second critical field H.sub.C2. In between critical fields H.sub.C1 and H.sub.C2, however, Type II superconductors exhibit partial flux exclusion. Partial flux exclusion is believed to be caused by inhomogeneities (e.g., pores, inclusions, grain boundaries) inside the Type II superconductor. Some of the magnetic flux lines become "pinned" in the superconducting material while others are repelled by the pinned flux lines. This repulsion causes levitation. Thus, levitation for a Type II superconductor does not arise from the Meissner effect. Instead, levitation occurs because the Type II superconductor behaves more like a perfect conductor than a Meissner conductor.
When the magnetic field is being induced into the Type II superconductor, the superconductor offers resistance to change or displacement of the pinned flux lines. This resistance allows the bearing made from Type II superconducting material to display far greater stability than a bearing made of a Type I superconducting material. Because of their inherent stability, Type II superconducting bearings are more commonly used for rotating structures.
For the superconducting bearing 2 shown in FIGS. 1a-1c, flux lines 4 flow between poles of a dipole permanent magnet 6, which is a distance x from a member 8 made of Type II superconducting material. When cooled below its critical temperature T.sub.C, the member 8 pins some of the flux lines 4. Other flux lines 4 are repelled by the pinned flux lines 4, causing the magnet 6 to be levitated above the superconducting member 8. When subjected to external loads F.sub.A, F.sub.B and F.sub.C, the magnet 6 is forced towards the surface of the superconducting member 8. Resulting from the interaction of flux lines 4 is a reactive, i.e., restoring, force F.sub.R which resists the motion of the magnet 6. As the external load increases, the reactive force also increases (see FIG. 2).
The bearing's load capacity is proportional to the magnet's flux density, which is defined as the amount of magnetic flux per unit area. Higher flux densities enable the bearing 2 to operate under greater loads. Load capacity can be increased by increasing flux density.
The bearing's stiffness is proportional to flux density gradient. Flux density gradient can be defined as the change of flux density from a distance x normal to the magnetic surface. It is proportional to the change in restoring force over distance x from the magnetic surface (dF/dx). The slope of the line in FIG. 2 indicates the flux density gradient for the superconducting bearing 2 shown in FIGS. 1a-1c. Larger negative slopes indicate higher bearing stiffness.
High bearing stiffness is often desirable to maintain accurate rotor positioning. When the rotor is deflected from its geometric axis, it must be restored. Bearing stiffness can be increased by increasing the flux density gradient.